The aim of the present study was to evaluate the cecal gas production (GP) and methane (CH4) production as well as cecal fermentation kinetics when corn grain (CG) was replaced with soybean hulls (SHs) in horse diets in the presence of different levels of Salix babylonica (SB) extract. Corn grains were replaced with SH at different levels (/kg): 0 g (control), 75 g (SH75), or 150 g (SH150), with the inclusion of SB extract at: 0, 0.6, 1.2, and 1.8 mL/g dry matter (DM) of substrates. Ration type × extract dose interactions were observed for GP and CH4 production at some incubation hours. Diets containing SH, without the inclusion of SB extract, increased the asymptotic GP (P = .031) and decreased (P < .01) the rate of GP and lag time of GP. The inclusion of SB increased (P = .009) the rate of GP, without affecting the asymptotic GP or lag time of GP. Besides, the SH-containing rations decreased (P < .05) CH4 production, with no effect for SB extract dose. The SH75 ration increased (P < .05) cecal fermentation pH, metabolizable energy, short chain fatty acids, and gas yield at 24 hours of incubation, but quadratically decreased partitioning factor at 24 hours of incubation (P = .023), whereas SB extract dose had no effect. It is concluded that SH-containing rations had higher potential fermentation efficiency and fermentation kinetics superior to that of CG. The level of 75 g SH/kg DM was the best level of inclusion to replace 30% CG in the diets of horses. The inclusion of SB extract did not affect the cecal fermentation kinetics of horse diets containing SH at different levels.